Meaning
Deformation speed in thermoplastic shaping influences the orientation and alignment of polymer chains. A high strain rate describes a condition where the material undergoes rapid deformation within a very short timeframe. In stretch blow moulding, this regime dictates how the polymer stretches under the combined action of the mechanical rod and the high-pressure air.
Molecular Response
Rapid drawing prevents polymer chains from relaxing during the stretch phase, forcing them into a highly aligned state. Under a high strain rate, the molecular network undergoes strain-induced crystallization, which increases the mechanical strength of the final product. This oriented structure is responsible for the barrier properties and clarity of the container.
If the deformation happens too slowly, the chains relax, resulting in a weak, amorphous structure.
Process Relevance
The speed of the stretch rod and the pressure rise of the air together determine the rate of deformation. Modern high-speed blow moulders operate at rates that exceed typical laboratory characterisation speeds. This disconnect makes accurate material modeling challenging without dynamic testing equipment.
Failure Risk
Stretching too fast can cause localized tearing or micro-cracking if the preform temperature is too low. The polymer becomes brittle when the rate of deformation exceeds its molecular stretching limit. Maintaining the preform temperature within the process window prevents these brittle failures during the rapid inflation phase.
